1. Field of the Invention
This invention relates to a dust filter unit for a canister of an evaporative fuel-processing system of an internal combustion engine.
2. Prior Art
A dust filter is generally employed in evaporative fuel-processing systems of internal combustion engines. FIG. 1 shows the arrangement of an evaporative fuel-processing system of an internal combustion engine, employing a conventional dust filter for a canister of the system. In the figure, a canister 226 which adsorbs evaporative fuel delivered from a fuel tank 223 has an open-to-atmosphere port 225 from which extends a drain passage 209 which opens into the atmosphere. Arranged across the drain passage 209 is a vent shut valve 252 for blocking the drain passage 209 during leakage checking of an evaporative emission control system including the fuel tank 223 and the canister 226, which is formed by an electromagnetic valve and electrically connected to an ECU 206 such that a valve element 252a thereof is moved in response to a driving signal from the ECU 206 to close a valve bore 253a formed in an end of a drain passage 253 connected to the drain passage 209. When no driving signal is output from the ECU 206, the valve element 252a is biased by a spring, not shown, in a position where the vent shut valve 252 is open.
The drain passage 253 extends from the vent shut valve 252 and is bifurcated into an inlet passage 254 and an outlet passage 255 which have respective one-way valves 258 and 259 arranged thereacross. These passages 254, 255 terminate in the atmosphere. When the pressure within the passage 253 is lower than the atmospheric pressure, the one-way valve 258 opens, while when the former is higher than the latter, the one-way valve 259 opens.
Arranged across an end of the inlet passage 254 opening into the atmosphere is a filter box 261 accommodating a drain filter 262 in the form of a cylinder formed of a meshed material, as a dust filter. Fresh air is introduced through the end of the inlet passage 254, passes through the cylindrical body of the drain filter 262 via an outer peripheral surface thereof into the interior of the filter 262 and then flows toward the one-way valve 258.
The drain filter 262, which is accommodated in the filter box 261 arranged across the inlet passage on the atmosphere side of the vent shut valve 252 arranged across the drain passage 209, filters out alien substances such as dust and dirt so that clean fresh air is supplied to the canister 226. As a result, failure of the valve element 252a to fully close the valve bore 253a in the passage 253 due to the presence of alien substances mixed in fresh air introduced from the atmosphere; can be avoided even if the vent shut valve 252 is activated, when leakage checking of the canister 226 and its associated passages is carried out.
In the conventional evaporative fuel-processing system including the dust filter, however, the one-way valve 259 is arranged across the outlet passage 255 branching off from the drain passage 253 on the atmosphere side of the vent shut valve 252, while the other one-way valve 258 and the filter box 261 are arranged across the inlet passage 254. Further, these valves and filter box are formed of separate pieces. This requires a large mounting space and imposes limitations on the layout of the passages in an automotive vehicle.